1. Field
The present disclosure relates generally to intelligent electronic devices (IEDs) for electrical power systems, and more particularly, to an intelligent electronic device having circuitry for reducing the burden placed on at least one current sensor of the device resulting in a highly accurate measurement by the at least one current sensor.
2. Description of the Related Art
Electric utility companies (“utilities”) track electric usage by customers by using power meters, also known as IEDS. These meters track the amount of power consumed at a particular location. These locations range from power substations, to commercial businesses, to residential homes. The electric utility companies use information obtained from the power meter to charge its customers for their power consumption, i.e., revenue metering.
A popular type of power meter is the socket-type power meter, i.e., S-base or Type S meter. As its name implies, the meter itself plugs into a socket for easy installation, removal and replacement. Other meter installations include panel mounted, switchboard mounted, and circuit breaker mounted. Additional meter forms include switchboard drawout forms, substation panel metering forms, and A-base front wired forms. Typically, the power meter connects between utility power lines supplying electricity and a usage point, namely, a residence or commercial place of business.
A power meter may also be placed at a point within the utility's power grid to monitor power flowing through that point for distribution, power loss, or capacity monitoring, e.g., a substation. These power and energy meters are installed in substations to provide a visual display of real-time data and to alarm when problems occur. These problems include limit alarms, breaker control, outages and many other types of events. Conventionally, the visual display includes numerical information and/or an alarm indication, e.g., a LED, on the face of the meter.
To determine power consumed in a system, a power meter measures or senses current and voltage of at least one phase of the power distribution system. Sensors employed in such systems include current sensors, current transformers (CTs), etc. for measuring current and voltage transformers, potential transformers (PTs), etc. for measuring voltage. Conventional sensors have an instability problem in transferring a measured signal from the primary side of a transformer to the secondary side. This behavior is inherent to current transformers and is directly related to the burden placed on the transformer, i.e., the value of the load resistance on the secondary winding. The instability is responsible for phase response shifts due to aging, temperature or mechanical stress of the transformer core. The aforementioned conditions can cause unpredictable errors in the power measurements at lower power factor conditions.
Existing measurement circuits in power meters rely on the quality of the CT's core material for stable signal transfer and only a narrow tolerance range is accepted. These circuits also yield limited dynamic range for the required accuracy performance. Techniques using post gain switching, as shown in FIG. 1, give marginal improvement due to the amplification of noise and distortion already present on the usable signal Isec at the output of the CT, across the load resistor Rload.
Therefore, there is an increasing demand in the electrical metering industry for devices, such as electrical power meters, analyzers, etc., to be able to measure AC current and its related parameters, e.g., power, energy, power factor, etc., with higher dynamic range and higher precision.